1. Field of the Invention
The present invention relates to an over-current protection apparatus, and more particularly, to an over-current protection apparatus with a current-sensitive element made by a positive temperature coefficient (PTC) conductive composition.
2. Description of the Prior Art
Since the portable electronic products (such as cellular phones, notebook computers, hand-held cameras and personal digital assistants, etc.) are getting more and more popular nowadays, over-current protection apparatuses for avoiding the portable electric products from over current or over temperature have become more and more important.
There are many kinds of conventional over-current protection apparatuses, such as a thermal fuse, bimetal or a positive temperature coefficient (PTC) over-current protection apparatus, etc. Nowadays, the PTC over-current protection apparatus has been widely applied for protecting batteries, especially secondary batteries, such as a nickel-hydrogen battery or a lithium battery, etc., due to its characteristics of being resettable, sensitive to temperature and reliable.
A PTC conductive composition acts as a current-sensitive element of the PTC over-current protection apparatus due to its temperature-sensitive resistance. The resistance of the PTC conductive composition is very low at a normal temperature so that the battery can operate normally. However, if an over-current or an over-temperature situation happens due to improper usage of the battery or other irregular cause, the resistance of the over-current protection apparatus will immediately increase for at least ten thousand times (such as over 104 ohm) to a high resistance state. Therefore, the over current will be counterchecked and the objective to protect the circuit element of the battery is achieved.
Generally, the PTC over-current protection apparatus needs to have the following properties:
1. Low resistance: Even if the battery is discharged normally, a sudden and large current may occur according to the requirement of the electronic equipment. At this time, a voltage drop will happen if the resistance of the PTC over-current protection apparatus is too high, and the operation of the electronic equipment will be disturbed thereby. Therefore, the resistance of the PTC over-current protection apparatus needs to be less than 30 milliohm in a normal condition, preferably less than 20 milliohm.
2. Low switching temperature: When the temperature of the battery is over a threshold, the resistance of the PTC over-current protection apparatus will suddenly raise to a high resistance state, which is called xe2x80x9cswitching temperature.xe2x80x9d Generally speaking, the switching temperature of the PTC over-current protection apparatus needs to be less than 100xc2x0 C. for preventing the battery from over-heat or burning down.
The PTC conductive composition is a crystalline polymer mixed with a conductive filler (such as carbon black or metal powder). Next, the PTC conductive composition is irradiated to perform a cross-linking reaction. The PTC conductive composition has a very low resistance at a room temperature. However, if the temperature rises over the switching temperature, the PTC conductive composition will switch to a high-resistance state immediately.
U.S. Pat. No. 5,801,612 discloses an over-current protection apparatus having a PTC conductive composition composed of conductive filler mixed polymer. The polymer applied in the above patent is a copolymer of polyolefin and polyacrytic acid. The PTC conductive composition is laminated with two electrodes and then irradiated to perform a cross-linking reaction. More specifically, the melting point of the polymer disclosed in the above patent needs to be less than 110xc2x0 C.; the crystallinity of the polymer needs to be less than 40%, and the cross-linking level equivalent of the irradiation of the PTC conductive composition needs to be less than 20 Mrads. Therefore, the above over-current protection apparatus is capable of sensing the current change in a low temperature range (i.e.,  less than 110xc2x0 C.) to provide a resistance switch for protecting the circuit and the battery. U.S. Pat. No. 5,580,493 and U.S. Pat. No. 5,378,407 also disclose a PTC conductive composition having a polymer made of a copolymer of polyolefin and polyacrylic acid. However, since the polyacrylic acid has a moisture absorption property, the stability of the PTC conductive composition containing acrylic-modified polymer will thereby decrease.
A major objective of the present invention is to provide an over-current protection apparatus having a current-sensitive element being at a low-resistance state when the temperature of the battery is normal; however, if the temperature of the battery raises to its switching temperature, the resistance of the current-sensitive element will increase immediately to a high-resistance state to countercheck the over current or over temperature of the battery.
Another objective of the present invention is to provide a method for rapidly and massively manufacturing a stable and temperature-sensitive over-current protection apparatus.
In order to achieve the above objective and to avoid the disadvantages of the prior art, the present invention discloses an over-current protection apparatus comprising a current-sensitive element and at least two electrodes. The current-sensitive element is composed of a positive temperature coefficient (PTC) conductive composition including at least one polymer, a conductive filler and a non-conductive filler: characterized in that the melting point of the polymer is greater than 110xc2x0 C. and the vicat softening point is less than 110xc2x0 C. for improving the conductivity and the sensibility to temperature of the over-current protection apparatus.
The present invention further discloses a method for manufacturing the over-current protection apparatus, comprising step (a) to step (d). In step (a), at least one polymer, conductive filler and non-conductive filler are well mixed to form a PTC conductive composition. In step (b), the PTC conductive composition is laminated with two electrodes to form a PTC sheet. In step (c), the PTC sheet is irradiated and has a cross-linking level equivalent to at least 20 Mrads, thus the PTC conductive composition will perform a higher degree of cross-linking reaction. In step (d), the PTC sheet is cut and the over-current protection apparatuses are formed thereby.
The foregoing and other objective and advantages of the invention and the manner by which the same is accomplished will be clearly shown in the following detailed description taken in conjunction with the accompanying drawings.